Ruby：分组并计算结果数

Question

如何在不获取数据的情况下计算“组”查询返回的结果数？到目前为止，我只是得到了结果的哈希表。 rails3中是否可以优化此查询？

Vote.group("question_id, user_id").where("question_id = 3").count.count
 => 2

在这种情况下，我们正在计算此哈希表=＆gt; {1=>10, 15=>1}

查询是：

SELECT COUNT(*) AS count_all, question_id, user_id AS question_id_user_id
FROM `votes`
WHERE (question_id = 3)
GROUP BY question_id, user_id

Answer 1

您可以使用count_by_sql：

Vote.count_by_sql("select count(*) from ( select 1 from Votes group by question_id, user_id )")

或者，您可以使用Rails构建查询，然后运行它：

 query = Vote.group(:question_id, :user_id).to_sql
 count = Vote.count_by_sql("select count(*) from ( #{query} )")